The Basque separatist group Eta is probably behind the robbery of 350 pistols from a French arms store, Spanish prime minister José Luis Rodríguez Zapatero said last night.

The robbery raised concerns that the group was about to return to terrorism some six months after it declared a "permanent" ceasefire.

Three masked raiders kidnapped the grandchildren and stepdaughter of the manager of an arms warehouse near Nîmes and then forced the manager to open it. "Everything leads us to believe that it was Eta," said the Nîmes public prosecutor, Robert Gelli.

He cited "the mode of operation, the presence of a woman, the foreign accent of the individuals ... and of course the nature of the objects stolen".

"It is probably Eta," Mr Zapatero admitted. " It is a grave and serious matter that will, eventually, bring consequences but we are not going to act precipitously."

Mr Zapatero's socialist government is committed to a peace process without cross-party support. The process has run into difficulties, with some 40 members of the banned Batasuna separatist political party formally accused of being Eta members. Mr Zapatero has warned that the road to peace will be long and difficult.

In a campaign aimed at creating an independent Basque state, Eta has killed more than 800 people in four decades.